Surface patterning by selectively depositing, removing, or otherwise influencing portions of surface material plays an important role, e.g., in the manufacture of integrated circuitry on silicon and other semiconductor devices as well as in the manufacture of magnetic bubble devices. Currently, such patterning typically requires the resolution of features as small as a few micrometers, and higher resolution is being sought for future devices. Patterning may be by one of a variety of optical, X-ray, ion beam, or electron beam lithographic methods such as, e.g., contact, proximity, or projection lithography; an alternative, known as "direct writing", involves pattern delineation by controlled positioning of a narrowly focused beam of radiation. (In current practice, this latter method is primarily used for making a first or "master" mask by electron-beam direct writing.)
A pattern is typically delineated, either by direct writing or by replication of a mask pattern, in a device-supported layer of "resist" material and, after exposure to actinic radiation, portions of this material are selectively removed in a development step. Depending on whether a resist material is "positive acting" or "negative acting", either exposed or unexposed portions are removed, respectively. Then, in a fabrication step, exposed device surface portions are preferentially affected.
Instead of a single, homogeneous layer of resist material, multiple-layer structures may be used for various reasons. For example, to facilitate removal of residual resist material after fabrication, a two-layer or three-layer structure may be advantageous as disclosed, e.g., by L. T. Romankiu et al., "Interlayer Technique for Use to Form Evaporation Lift-Off Masks", IBM Technical Disclosure Bulletin, Vol. 13, No. 12, May 1976, pp. 4219-4221.
Multi-layer structures have also been designed for enhanced resolution of detail as resulting from more faithful replication of a desired pattern in a resist layer. For example, as described by J. M. Moran et al., "High Resolution, Steep Profile Resist Patterns", The Bell System Technical Journal, Vol. 58, 1979, pp. 1027-1036, a topmost layer may serve as a mask for replicating a pattern in an underlying second layer which, in turn, serves as a dry processing mask for a relatively thick bottom layer of a partially polymerized organic material. The thickness of this third layer is chosen sufficient, e.g., to afford coverage of steps in a surface to be patterned while presenting an essentially flat surface to the defining radiation. It is an advantage of this approach that, due to the flatness and opacity of the underlying polymer layer, standing waves of actinic radiation do not cause undue loss of resolution.
Another double layer resist system has been proposed by B. J. Lin, "Portable Conformable Mask--A Hybrid Near-Ultraviolet and Deep-Ultraviolet Patterning Technique", Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 174: Developments in Semiconductor Microlithography IV (1979), pp. 114-121. Use of this system involves repeated exposure and development steps affecting diazo-containing and poly(methyl methacrylate) materials.